1. Field of the Invention
The present invention relates generally to the field of semiconductor manufacturing and particularly to characterization of light sources utilized in ULSI lithography.
2. Background
Photolithography plays a vital role in semiconductor manufacturing by defining the ultimate features that are etched or deposited within each layer of the device. Projection imaging machines, usually of the stepper (see, for example, Bruning et al., “Optical Lithography—Thirty Years and Three Orders of Magitude”, Proc. of SPIE, Vol. 3051, pp. 14-27, 1997) or step and scan (see, for example, “Optical Lithography—Thirty Years and Three Orders of Magnitude”, supra) variety, typical use effective light sources that can be varied over a wide range of configurations. FIG. 1 shows a block diagram of a typical projection imaging system as would be found in a stepper or scanner. Effective source, ES, is responsible for generating and shaping the light incident on the reticle. It includes a light source, LS, (typically an excimer laser), two blocks of beam shaping optics (IIO and OIO) that produce spatially and angularly uniform light incident on reticle R. The spatial uniformity requirement is simple; it must be constant across reticle R. Angular uniformity means the angular spectrum of radiation (dE/do(nxr,nyr)) needs to be the same at all field points (e.g., x,y transverse positions on R). FIG. 2a shows in plan view the angular distribution (direction cosine coordinates nx, ny shown) of a typical source (annular quadrupole). The exact angular distribution of light within this source, the radiant intensity or dE/do(nx,ny) (energy per unit solid angle), can be measured by in-situ methods according to see, for example, U.S. Pat. No. 6,356,345, McArthur et al. “InSitu Source Metrology Instrument and Method of Use” McArthur et al., “In-Situ Source Metrology Instrument and Method of Use”, U.S. Pat. No. 6,741,338, May 25, 2004, and Smith et al., “Apparatus and Method for High Resolution In-Situ Illumination Source Measurement in Projection Imaging Systems”, U.S. Patent Publication No. U.S.20050231705, Oct. 20, 2005.
As photolithography pushes to k1 values below the well known Rayleigh resolution limit (k1=0.5) image enhancement techniques among which are polarized sources become crucial (see, for example, Sheppard et al., “Annular pupils, radial polarization, and superresolution”, Applied Optics, Vol. 43, No. 22, pp. 4322-4327, Aug. 1, 2004). The effective source in FIG. 2a is polarized for enhanced resolution. Poles P2 and P4 consist of linearly polarized light in the X direction (indicated by direction of arrows in these poles) while poles P1 & P3 are Y polarized. Again, for low k1 semiconductor applications the state of polarization of the effective source is critical to lithographic performance since imaging contrast for X and Y (p and s) polarization states differ dramatically as the numerical aperture of the optical system is increased above ˜0.7 (see, for example, Smith et al., “Challenges in high NA, polarization, and photoresists”, Proc. of SPIE, Vol. 4691-2, pp. 11-24, 2002). Therefore it is desirable to have a technique that could determine the state of polarization of the effective source in a projection tool.
Exposure Sources; General Description
Quite generally, light exiting from an excimer laser or filtered mercury source, is typically described in terms of the exposure wavelength, bandwidth, and polarization. For example, 248 nm excimer laser sources have FWHM bandwidths (full wave half maximum) approaching 0.1 pm and depending on application can take on several different polarization states. Following the laser (or arc lamp) the exposure light is again modified (polarized or un-polarized) and shaped for imaging using custom illuminators. For most applications the polarization of the light impinging on the reticle is un-polarized although the industry is rapidly making use of polarized source configurations as discussed above. However, since laser beams (or filters) are never monochromatic and due to various interactions of the beam with the optical system, the polarization state of any light source is never perfect and always contains a mixture of polarized and un-polarized light; by un-polarized we mean a component that can be represented as a superposition of waves with a rapidly varying succession of different polarization states.
Polarized Light
Here, we will be mainly concerned with both polarized and un-polarized exposure sources. Polarized (or un-polarized) light can be completely characterized by four Stokes parameters S0, S1, S2, S3 (see, for example, Born et al., “Principles of Optics, Coherence Matrix and Stokes Parameters”, Principles of Optics, 7th (expanded) Edition, pp. 619-632, 1999) or equivalently by it's 2×2 polarization matrix (see, for example, “Principles of Optics, Coherence Matrix and Stokes Parameters”, supra) (sometimes called a coherence matrix) Pij:Pij=<Ei*conjg(Ej)>=|P11P12|(i,j=1:2)  (Equation 1)|P21P22|where Ei is the time varying electric field component,  denotes time averaging, and conjg( ) denotes complex conjugation. P has four components which we hereafter utilize as a four vector:P=(P1,P2,P3,P4)=(P11,P22,Re(P12), Im(P12))  (Equation 2)Where Re/Im denote real and imaginary parts. In our apparatus, various components will be employed to further polarize or otherwise alter the state of the incident. The physical polarizing action of these components is most concisely expressed using Jones matrices (see, for example, Hecht, “Polarization”, Optics, Third Edition, Chapter 8, pp. 319-376, 1998). To better understand this invention it is helpful to review some concepts and notation concerning the mathematical representation of polarization (see, for example, “Principles of Optics, Coherence Matrix and Stokes Parameters”, supra, “Polarization”, supra) and the interaction of the fields with material objects (lens, optical systems).The transverse electric field can be defined by Equation 3:Ej=ej(Δν)ei(kz−wt+Δνt) j=1,2  (Equation 3)Effect of polarizing element is to change ej→e′j where
                                          e            k            ′                    ⁡                      (                          Δ              ⁢                                                          ⁢              v                        )                          =                              ∑                          j              =              1                        2                    ⁢                                    J              kj                        ⁢                                          e                j                            ⁡                              (                                  Δ                  ⁢                                                                          ⁢                  v                                )                                                                        (                  Equation          ⁢                                          ⁢          4                )            where Jkj is the 2×2 Jones matrix.The effect of successive polarizing elements is accounted for by multiplying the Jones matrices, e.g.,
                                          e            l            ″                    ⁡                      (                          Δ              ⁢                                                          ⁢              v                        )                          =                                            ∑                              j                ⁢                                                                  ⁢                1                                      ⁢                          J              ⁢                                                          ⁢                              2                                  l                  ⁢                                                                          ⁢                  j                  ⁢                                                                          ⁢                  1                                            ⁢                              e                                  j                  ⁢                                                                          ⁢                  1                                ′                                              =                                                    ∑                                                      j                    ⁢                                                                                  ⁢                    1                                    ,                                      j                    ⁢                                                                                  ⁢                    2                                                              ⁢                              J                ⁢                                                                  ⁢                                  2                                      lj                    ⁢                                                                                  ⁢                    1                                                  ⁢                J                ⁢                                                                  ⁢                                  1                                      j                    ⁢                                                                                  ⁢                    1                    ⁢                    j                    ⁢                                                                                  ⁢                    2                                                  ⁢                                  e                                      j                    ⁢                                                                                  ⁢                    2                                                                        =                                          ∑                                  j                  ⁢                                                                          ⁢                  2                                            ⁢                                                                    (                                          J                      ⁢                                                                                          ⁢                      2                      *                      J                      ⁢                                                                                          ⁢                      1                                        )                                                        lj                    ⁢                                                                                  ⁢                    2                                                  ⁢                                  e                                      j                    ⁢                                                                                  ⁢                    2                                                                                                          (                  Equation          ⁢                                          ⁢          5                )            Equation 5 is diagrammatically shown in FIG. 2b where light with polarization vector e is incident first on optical element 1 with Jones matrix J1 and then on optical element 2 with Jones matrix J2, the result of all this being the net system; element 1 followed by element 2, is represented by the Jones matrix J3=J2*J1 which is the matrix product of J2 and J1.
The measurable quantities are intensities and the contribution of light color νc+Δν (νc=center or nominal frequency) is:
                              dI          =                                                    ∑                k                            ⁢                                                                    e                    k                    ′                                    ⁡                                      (                                          Δ                      ⁢                                                                                          ⁢                      v                                        )                                                  ⁢                                  e                  k                  ′                                *                                  (                                      Δ                    ⁢                                                                                  ⁢                    v                                    )                                ⁢                dv                                      =                                          ∑                                  k                  ,                                      j                    ⁢                                                                                  ⁢                    1                                    ,                                      j                    ⁢                                                                                  ⁢                    2                                                              ⁢                                                J                                      kj                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  J                                      kj                    ⁢                                                                                  ⁢                    2                                    *                                                                    ⁢                                  ⁢                                            e                              j                ⁢                                                                  ⁢                1                                      ⁡                          (                              Δ                ⁢                                                                  ⁢                v                            )                                ⁢                                    e                              j                ⁢                                                                  ⁢                2                            *                        ⁡                          (                              Δ                ⁢                                                                  ⁢                v                            )                                ⁢          dv                                    (                  Equation          ⁢                                          ⁢          6                )            Integrating over frequencies, the measured intensity is:
                              I          =                                                    ∑                                  K                  ,                                      j                    ⁢                                                                                  ⁢                    1                                    ,                                      j                    ⁢                                                                                  ⁢                    2                                                              ⁢                                                J                                      kj                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  J                                      kj                    ⁢                                                                                  ⁢                    2                                    *                                ⁢                                  P                                      j                    ⁢                                                                                  ⁢                    1                    ⁢                    j                    ⁢                                                                                  ⁢                    2                                                                        =                                                            ∑                                                            j                      ⁢                                                                                          ⁢                      1                                        ,                                          j                      ⁢                                                                                          ⁢                      2                                                                      ⁢                                  J                  *                                      S                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      j                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      P                                          j                      ⁢                                                                                          ⁢                      1                      ⁢                      j                      ⁢                                                                                          ⁢                      2                                                                                  =                              Trace                ⁡                                  (                                      JS                    *                    P                                    )                                                                    ⁢                                  ⁢                  where          ⁢                      :                                              (                  Equation          ⁢                                          ⁢          7                )                                                                                    P                                  j                  ⁢                                                                          ⁢                  1                  ⁢                  j                  ⁢                                                                          ⁢                  2                                            =                            ⁢                              〈                                                      e                                          j                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      e                                          j                      ⁢                                                                                          ⁢                      2                                        *                                                  〉                                                                                        =                            ⁢                              polarization                ⁢                                                                  ⁢                matrix                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                light                                                                                        =                            ⁢                              equivalent                ⁢                                                                  ⁢                to                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                Stokes                ⁢                                                                  ⁢                Parameters                                                                        (                  Equation          ⁢                                          ⁢          8                )                                          JS                      jj            ′                          =                                            ∑              k                        ⁢                                          J                kj                *                            ⁢                              J                                  kj                  ′                                                              =                                                    ∑                k                            ⁢                                                J                  jk                  t                                ⁢                                  J                                      kj                    ′                                                                        =                                                            (                                                            J                      t                                        ⁢                    J                                    )                                ⁢                                  jj                  ′                                            =                              Hermetian                ⁢                                                                  ⁢                matrix                                                                        (                  Equation          ⁢                                          ⁢          9                )            and
t denotes complex conjugate, transpose or Hermetian conjugate matrix.
The intensity (I) is expressed as:I=JS11*P11+JS22*P22+JS12P12*+JS12*P12  (Equation 10)writing I in terms of independent components of JS and P we get:
                    I        =                              [                                          JS                11                            ⁢                              JS                22                            ⁢              2              *                                                R                  e                                ⁡                                  (                                      JS                    12                                    )                                            ⁢              2              *                                                I                  m                                ⁡                                  (                                      JS                    12                                    )                                                      ]                    ⁡                      [                                                                                P                    11                                                                                                                    P                    22                                                                                                                                          R                      e                                        ⁡                                          (                                              P                        12                                            )                                                                                                                                                              I                      m                                        ⁡                                          (                                              P                        12                                            )                                                                                            ]                                              (                  Equation          ⁢                                          ⁢          11                )            
Now, if we do, a=1:n separate intensity measurements of the light using n different arrangements of the elements, then the net result written in matrix form is:
                              [                                                                      I                  1                                                                                    ⋮                                                                                      I                  a                                                                                    ⋮                                                                                      I                  n                                                              ]                =                                                       [                                                                                                                  JS                        11                        1                                            ⁢                                                                                          ⁢                                              JS                        22                        1                                            ⁢                                                                                          ⁢                      2                      *                                                                        R                          e                                                ⁡                                                  (                                                      JS                            12                            1                                                    )                                                                    ⁢                                                                                          ⁢                      2                      *                                                                        I                          m                                                ⁡                                                  (                                                      JS                            12                            1                                                    )                                                                                                                                                          ⋮                                                                                                                                      JS                        11                        2                                            ⁢                                                                                          ⁢                                              JS                        22                        2                                            ⁢                                                                                          ⁢                      2                      *                                                                        R                          e                                                ⁡                                                  (                                                      JS                            12                            2                                                    )                                                                    ⁢                                                                                          ⁢                      2                      *                                                                        I                          m                                                ⁡                                                  (                                                      JS                            12                            2                                                    )                                                                                                                                                          ⋮                                                                                                                                      JS                        11                        n                                            ⁢                                                                                          ⁢                                              JS                        22                        n                                            ⁢                                                                                          ⁢                      2                      *                                                                        R                          e                                                ⁡                                                  (                                                      JS                            12                            n                                                    )                                                                    ⁢                                                                                          ⁢                      2                      *                                                                        I                          m                                                ⁡                                                  (                                                      JS                            12                            n                                                    )                                                                                                                                ]                        ⁢                                                         [                                                                                                    P                        11                                                                                                                                                P                        22                                                                                                                                                                          R                          e                                                ⁡                                                  (                                                      P                            12                                                    )                                                                                                                                                                                                  I                          m                                                ⁡                                                  (                                                      P                            12                                                    )                                                                                                                    ]                                                                        (                  Equation          ⁢                                          ⁢          12                )            or
The elements of ID can be related to rows of a Mueller matrix (see, for example, Azzam et al., “Propagation of Polarized Light”, Elsevier Science B.V., ISBN 0 444 870164, 1999). Additional formulations can be found in see, for example, “Propagation of Polarized Light”, supra: Propagation of Polarized Light, R. Azzam, chapters 2 and 3.In-Situ Source Metrology
A technique that will be used in the practice of this invention is in-situ source metrology (see, for example, U.S. Pat. No. 6,356,345, supra and U.S. Pat. No. 6,741,338, supra and “Apparatus and Method for High Resolution In-Situ Illumination Source Measurement in Projection Imaging Systems”, supra). This is required for recovering the polarization matrix, P, as a function of incident light direction (P=P(nx,ny)). Referring to FIG. 1, it generally consists of replacing reticle, R, with attached pellicle PE with either an array of pinhole cameras (see, for example, U.S. Pat. No. 6,356,345, supra and U.S. Pat. No. 6,741,338, supra) (pinhole is either above or below the reticle face) or an array of in-situ source imaging objectives (see, for example, “Apparatus and Method for High Resolution In-Situ Illumination Source Measurement in Projection Imaging Systems”, supra). These three different arrangements are shown in FIG. 3 as ISMI1 (reticle backside pinhole), ISMI2 (pellicle plane pinhole), and ISMI3 (in-situ source imaging objective). Another configuration, (not shown) works in a reflective system as a transmissive pinhole array placed either prior to or after the reticle (blank, reflective reticle). In the following, the provided in-situ source metrology instrument (ISMI) will typically be combined with the polarizing elements into an integrated arrangement.
Ellipsometry
One common technique for characterizing the optical and solid-state properties of thin-films, surfaces, and material microstructure using polarized light is, ellipsometry. The basic principles (and apparatus) are described fairly easily with reference to FIG. 4a which shows a typical PCSA (Polarizer, Compensator, Sample, Analyzer) system—a more detailed discussion can be found in see, for example, “Propagation of Polarized Light”, supra. First, the polarization state of an incoming inspection light (L—FIG. 4a) is typically known and possibly adjustable through the use of a polarizer (P) and compensators (C). The input light (L) is then reflected (or transmitted through) from the material (S—FIG. 4a) of interest. The interaction of the light (L) with the sample interface causes a polarization (including possible changes in phase and amplitudes) change in the input, from linear to elliptical polarization (see above discussion on polarization). The polarization change or change in the ellipse of polarization is then measured by analyzing the light reflected from the sample using a system of analyzers and detectors (A and D in FIG. 4a). In general, ellipsometry measures two values, Psi and Delta, which describe the polarization change upon interaction with the sample or optical system. For reflective systems these two values are related to the ratio of Fresnel reflection coefficients, Rp and Rs for p- and s-polarized light, respectively. Material characteristics such as film thickness, optical constants, refractive index are found by using the measured values of Psi and Delta with various material (oscillator models for example) models and algorithms to produce a system of non-trivial equations that describe the interaction of light (change in polarization) with the sample.
Polarimetry
While ellipsometry is commonly used for determining the optical properties of bulk semiconductor materials, thick layers, thin-films, and optical components as described above, it is more generally defined as a technique that can measure the state of polarization of an electromagnetic vector wave (see, for example, “Propagation of Polarized Light”, supra)—also known as polarimetry. For our present discussion we will be interested in describing the novel ellipsometry/polarimetry configuration shown in FIG. 4b (compare with FIG. 4a); since, for our present work we will describe a method for determining the state of polarization of an input light source (L) using an arrangement of polarizing element groups (PEGs with known and possibly unknown properties), a reticle camera (C), and a optical system (O) with a known polarization and transmission properties (for example the Jones-pupil) for many different field points. Put another way, we seek to describe a method and apparatus for in-situ photolithographic polarimetry with the ability to determine the state of polarization of the input light source (L) as a function of direction cosine after passing through a reticle and optical system (PIO—possibly polarizing). The method of the present invention is illustrated in a flow diagram (FIG. 4c) and can easily be compared with FIGS. 4a and 4b in terms of the symbols just discussed.
It would be desirable to have a technique for determining the actual state of polarization of light incident on a reticle in a projection imaging tool that is capable of providing the polarization matrix resolved into both its transverse spatial (x,y) and angular (nx,ny) components (P=P(nx,ny;x,y)) without the use of phase shifting masks and time consuming calibrations. That is, there is a need in the art for improved methods and apparatus for determining a source polarization matrix.